Intermittent fasting (IF) is emerging as a compelling diet trend, hailed for its positive effects on metabolism, appetite and even weight loss.

Basically, IF means restricting your eating to a particular window of time. Popular IF intervals include the 5:2 diet, where you eat normally for five days then cut calories on two non-consecutive days; 16:8, where you only eat within an eight-hour window (say, between 11am and 7pm); and 12-hour overnight fasts.

But despite IF's rising popularity, there's still a lot of mysteries about it yet to be solved by science.

For example, researchers from the University of Alabama at Birmingham (UAB) previously demonstrated that a form of IF called early time-restricted feeding (eTRF) boosts metabolism and health — but is this just a flow-on effect from eating fewer total calories and subsequently losing weight, or because there's something special about IF itself?

For a small study published in Cell Metabolism, the UAB team sought to find out by recruiting eight men with prediabetes — higher than normal levels of glucose in their blood.

For five weeks, the men were assigned one of two diets: the eTRF schedule, where they were only permitted to eat within a six-hour window and had to finish dinner by 3pm at the latest; and a regular schedule where their meals were spread out within a 12-hour window.

The researchers provided the men all their meals and told them when to eat, balancing the calories so that both groups ate the same amount of food overall (the first time such care has been taken in an IF study).

After five weeks, the groups took a break to "reset" their bodies, then switched diets for a further five weeks.

The results were a boon for intermittent fasting: eTRF improved the men's insulin sensitivity (meaning their cells were better at mopping up the excess blood glucose) and how efficiently their pancreases supplied insulin, their blood pressure dropped "dramatically", as did their levels of oxidative stress.

And though you'd think you'd feel pretty hungry if you stopped eating for the day at 3pm, the men's evening appetite levels also lowered.

The UAB researchers believe eTRF delivered such positive results because it syncs up your eating with your circadian rhythms — the internal body clock that dictates how your body works, and when.

"If you eat late at night, it’s bad for your metabolism,” said study co-author Courtney Peterson, an assistant professor of nutrition sciences, in a statement. “Our bodies are optimised to do certain things at certain times of the day, and eating in sync with our circadian rhythms seems to improve our health in multiple ways.

"For instance, our body’s ability to keep our blood sugar under control is better in the morning than it is in the afternoon and evening, so it makes sense to eat most of our food in the morning and early afternoon."

IF is often touted for its weight-loss benefits, and following the earlier research Peterson said "eating only during a much smaller window of time than people are typically used to may help with weight loss".

May is an important word there, because the UAB studies are only conducted on small numbers of people over short periods — the long-term benefits of intermittent fasting are yet to be conclusively proven. The UAB noted that larger studies need to be conducted that include women, and explore other forms of IF.